For Salted Caramel: In a medium sauce pan set to medium-high heat, combine sugar and water. Do not stir. Watch carefully until mixture turns a deep amber color. Remove from heat and whisk in butter, cream and salt. Set aside.
For Rice Krispie Treats: In a large pot set to medium heat, melt marshmallows, butter and vanilla. Stir until smooth and add salted caramel. Stir in Rice Krispies until well combined. Pour mixture into a lined and greased 13x9 (or 8x8 for thicker treats) pan. Flatten using a buttered rubber spatula. Chill pan in refrigerator while you make the ganache.
For Chocolate Ganache Topping: In a small pot over medium heat, heat cream until almost boiling. Remove from heat and whisk in chocolate until smooth. Let ganache cool for 10 minutes then pour over Rice Krispies, tilting the pan to distribute evenly. Let set for 5 minutes then sprinkle with coarse salt.
Refrigerate to set before cutting, at least 2 hours. Slice into squares and refrigerate until ready to serve.